Coffee is brewed by placing hot water into contact with ground, roasted coffee for a given period of time. Coffee beans contain approximately 600 chemical compounds, a high percentage of which result in bitter and poor tasting coffee, which is often incorrectly referred to as "strong coffee." These undesirable compounds are normally released into brewed coffee when the hot water remains in contact with the ground coffee beans for relatively long durations. A primary influence on the taste of brewed coffee is therefore the "brew time," the amount of time that the heated water is in contact with the ground coffee beans. Brew time is similarly important to the brewing of teas.
Experts in the coffee industry recommend brewing so-called "regular grind" coffee using brew times within the range of two to four minutes, where the brew time is the amount of time for which heated liquid is in contact with the ground beans. For free grinds (often called "espresso grinds"), brew times of approximately 20 to 30 seconds are recommended. It is also recommended that water which has been heated to approximately 205.degree. F. be used for brewing coffee.
In a typical coffee maker available for commercial and consumer use, coffee grounds are held in a paper filter which rests inside a cone-shaped plastic or metal chamber. An aperture having fixed dimensions is formed at the bottom of the chamber. Heated water (which in the prior art is usually only heated to between 170.degree. and 185.degree. F.) is pumped onto the coffee grounds in the plastic chamber at a relatively constant rate (which, as discussed below, is too low and thus results in a brew time that is too long). Brewed coffee flows from the brewing chamber through the aperture, and accumulates in a receptacle positioned below the brewing chamber. The rate at which the brewed coffee flows out of the brewing chamber is relatively constant and is dependent upon the diameter of the aperture and the flow restriction caused by coffee accumulation on the filter surface.
Because this prior art coffee maker pumps heated water onto the coffee grounds and discharges brewed coffee from the brewing chamber at constant flow rates, the amount of time during which the liquid is in contact with the coffee grounds is proportional to the amount of coffee to be brewed. For example, if a large quantity of coffee is to be brewed, the large volume of brewed coffee will take longer to flow into and drain from the brewing chamber than would a smaller volume. The brew time for a volume of four cups of coffee is thus approximately one-third the time of that for twelve cups. The prior art coffee maker therefore maintains contact between the coffee grounds and the liquid for durations that are much longer time than those recommended. Amongst currently available coffee makers, these durations range from approximately eight minutes to 15 minutes for 12 cups, where a "cup" of coffee is normally approximately 5 fluid ounces.
Prior art coffee makers utilize various mechanisms for pumping heated water in the brewing chamber. One such prior art coffee maker utilizes a "thermal pump" to heat water and deliver the heated water into the brewing chamber.
To brew coffee in a thermal pump device, a user fills a reservoir in the coffee maker with cool water. The water flows downwardly from the reservoir through flexible tubing, then through a check valve, and finally into a metal tube which is heated by a hot plate that also supports a coffee pot. A vertically-oriented plastic tube is continuous with the heated tube and has a discharge end which extends into the brewing chamber.
When the reservoir is filled with water, both the heated tube and the plastic tube become filled with a column of water. The user activates a power switch to begin brewing, causing the heating tube to heat the water inside it to the point of boiling. The boiling water generates a steam bubble which rises and pushes the column of water in the vertical section of tube upwardly, until the column of water flows out the discharge end of the tube and flows over the ground coffee. As the water exits the tube, the pressure on the bubble is reduced. The bubble thus expands rapidly and pushes the column of water out in a short burst. The check valve prevents the steam bubble from pushing the water back into the reservoir.
Once the steam bubble has exited the tube, additional water enters the tube from the reservoir and the cycle repeats. This system produces a "pulsed" flow which has a relatively constant flow rate averaged over the total flow time.
Another currently available coffee maker employs thermal pulse technology in combination with a diverter valve that diverts a portion of the heated water directly into the coffee pot below the brewing chamber, while the remainder of the heated water is directed into the brewing chamber. The diverted water dilutes the brewed coffee and reduces its perceived "strength." This method, which is sometimes referred to as the 80/20 method since approximately 20% of the heated water is diverted, is found in brewing systems sold for commercial use as well as those sold for the home.
A third type of currently available coffee brewing system has a pumping mechanism which utilizes a heated vessel similar to a residential hot water heater. At all times, water having a temperature of 200.degree. F. is held in the heated vessel. To brew coffee, a user pours the appropriate mount of cool water through an opening on the top of the machine. The cool water flows through a tube into the heated vessel and displaces the heated water, causing it to exit from the heated vessel via an outlet positioned over the coffee grounds. The cool water poured into the heated vessel subsequently becomes heated to 200.degree. F. in preparation for the next coffee-making cycle.
The heated vessel system is advantageous over the thermal pulse system in that the rate at which heated water flows onto the coffee grounds in the heated vessel system is substantially higher. For example, 10 cups of coffee are typically brewed in approximately four minutes.
Although each of the above-described coffee brewing systems are somewhat satisfactory for brewing small quantifies of coffee, it has been found that the taste of coffee brewed using those systems is inconsistent over the range of volumes of coffee that those systems can produce. It has been discovered that coffee having consistent flavor can be produced, regardless of the volume of coffee being brewed at one time, if the brew time used in the brewing process is substantially the same for any volume of coffee being brewed in the system. Each of the prior art systems described above lacks a feature which will maintain a consistent brew time regardless of the quantity of coffee to be made in the system. A new coffee making system which utilizes a consistent brew time is therefore desirable.